The invention relates to water soluble prodrugs in which the solubulizing groups are non toxic acids which are attached to paclitaxel as an ester functionality to the C2xe2x80x2- and/or C7-hydroxyl position. These prodrugs are stable in aqueous solution but are readily hydrolyzed at physiological conditions to the parent drug.
Paclitaxel (1) is a natural diterpenoid, isolated from the Pacific yew tree (Taxus brevifolia). Paclitaxel has been approved for treatment of patients with advanced ovarian cancer or breast cancer. 
Although paclitaxel has demonstrated to be an unique antitumor agent, it has several disadvantages. One of the major problems is its poor solubility in water, which makes formulation difficult in relation to the intravenous administration. Due to this poor solubility, paclitaxel is formulated, using a 1:1 mixture of cremophor EL (a polyethoxylated castor oil) and ethanol (Rowinsky, E. K. et al. J. Natl. Cancer Inst. 1990, 82, 1247). This mixture is diluted with 5% dextrose in water or saline prior to lengthy infusion. Unfortunately, various hypertensive reactions have been reported in patients who were treated with paclitaxel, partly due to cremophor EL, which is responsible for histamine release, causing the effects (Rowinsky, E. K. et al. Ibid.). Premedication, using antihistaminic drugs can deminish these side effects, but results in additional medication, cost and discomfort to the patient.
The solubility problems with paclitaxel could be overcome by the development of a more water soluble, chemically stable, and therefore more easily formulated analog/prodrug of paclitaxel. Prodrug strategies consist of temporary modification of the physiochemical properties of a compound through chemical derivatization. Such temporary chemical modification is usually designed to alter aqueous solubility and biodistribution while the pharmacological properties of the parent drug remain intact. Prodrugs can be designed to be converted in a predictable way, in vivo, to the active drug by either an enzymatic mechanism or by hydrolysis initiated under physiological pH conditions.
SAR studies have shown that some modifications at C7 of paclitaxel are allowed ((a) Mellado, W. et al. Biochem. Biophys. Res. Comm. 1984, 124, 329-336. (b) Kingston, D. G. I et al. New Trends in Nat. Prod. Chem. 1986, 26, 219-235. (c) Horwitz, S. B. et al. Ann. New York Acad. Sci. 1986, 466, 733-740. (d) Kingston, D. G. I. et al. J. Nat. Prod. 1990, 53, 1-12. (e) Ringel, I. et al. J. Pharmacol. Exp. Ther. 1987, 242, 692-698. (f) Chaudhary, A. G. et al. J. Org. Chem. 1993, 58, 3798-3799. (g) Chen, S. et al. J. Org. Chem. 1993, 58, 5028-5029).
For instance, 7-acetylpaclitaxel has shown to be as active as paclitaxel in microtubule assembly assays. SAR studies have also shown that introduction of an acetyl group at C2xe2x80x2 resulted in the loss of the ability to promote microtubule assembly. However the cytotoxic activity of 2xe2x80x2-acetyl-paclitaxel is almost the same as for paclitaxel, probably due to the fact that the C2xe2x80x2-acetyl group is either being hydrolyzed under the conditions of the bioassay or converted intracellularly to paclitaxel or an active paclitaxel metabolite. These observations suggest that the C2xe2x80x2- and C7-positions of paclitaxel are suitable for (temporary) structural modifications. The C2xe2x80x2-position seems more suitable for reversible derivatization.
Several research groups have reported the syntheses and biological evaluations of water soluble prodrugs of paclitaxel. These analogs have a polar substituent either at the C2xe2x80x2- or at the C7-hydroxyl group. In most cases the polar substituents are coupled to these hydroxyl groups via an ester, carbonate or carbamate functionality. Deutsch et al. (Deutsch, H. M. et al. J. Med. Chem. 1989, 32, 788-792) reported that some salts of 2xe2x80x2-succinylpaclitaxel and 2xe2x80x2-glutarylpaclitaxel had improved antitumor activities compared to the corresponding free acids. The triethanolamine and N-methyl-glucamine salts showed improved aqueous solubility and were more active than the sodium salts. Zhao et al. (Zhao, Z. et al. J. Nat. Prod. 1991, 54, 1607-1611) introduced sulfonate groups to improve the water solubility of paclitaxel. These sulfonate-paclitaxel analogs showed improved water solubility and had about the same (in vivo) activity compared to paclitaxel. Mathew et al. (Mathew, A. E. et al. J. Med. Chem. 1992, 35, 145-151) reported the synthesis and evaluation of some 2xe2x80x2- and 7-amino acid analogs of paclitaxel. The methane sulfonic salts of both 2xe2x80x2- and 7-amino acid esters of paclitaxel showed increased water solubility. The 2xe2x80x2-analogs showed activity to an extent similar to that of paclitaxel, while the others showed reduced activity. Vyas et al. (Vyas, D. M. et al. Bioorg. Med. Chem. Lett. 1993, 3, 1357-1360) synthesized and evaluated 2xe2x80x2- and 7-phosphate paclitaxel analogs. These analogs showed improved water solubility. However in vitro as well as in vivo these derivatives were non-toxic compared to paclitaxel. Ueda et al. (Ueda, Y. et al. Bioorog. Med. Chem. Lett. 1993, 3, 1761-1766) synthesized 2xe2x80x2- and 7xe2x80x2-phosphonoxyphenylpropionatepaclitaxel, which both showed increased water solubility. The 2xe2x80x2-analog was inactive, whereas the 7-analog was as active as paclitaxel in vivo. Greenwald et al. ((a) Greenwald, R. B. et al. J. Org. Chem. 1995, 60, 331-336. (b) Greenwald, R. B. et al. J. Med. Chem. 1996, 39, 424-431) prepared some 2xe2x80x2- and 7-polyethyleneglycol esters of paclitaxel. These analogs were extremely water soluble. The 2xe2x80x2-analogs had in vitro and in vivo activities in the same extent as paclitaxel, whereas the 7-analogs showed reduced activity. Greenwald et al. claimed that by choosing the appropriate weight for the 2xe2x80x2-PEG moiety, a prodrug was produced that is as efficacious as paclitaxel/cremophor EL/ethanol in an in vivo model. Nicolaou et al. (Nicolaou, K. C. et al. Nature 1993, 364, 464-466) synthesized some 2xe2x80x2-(2-thio-aryl)ethylcarbonate analogs of paclitaxel as well as some 2xe2x80x2- and 2xe2x80x2,7-(bis)C(O)CH2XCH2COOH (wherein X=O, S or SO2) analogs of paclitaxel, which were all more water soluble and showed increased in vitro cytotoxic activities compared to paclitaxel. Nicolaou et al. ((a) Nicolaou, K. C. et al. Angew. Chemie 1994, 106, 1672-1675. (b) Paloma, L. G. et al. Chem. Biol. 1994, 1, 107-112) also synthesized 2xe2x80x2- and 7-methylpyridiniumacetate analogs of paclitaxel. Both compounds showed increased water solubility. The 2xe2x80x2-analog was as active as paclitaxel in in vivo models, whereas the 7-analog was far less cytotoxic. Kingston et al. (Kingston et al. US patent 1995, U.S. Pat. No. 5,411,984A) prepared some 2xe2x80x2- and 2xe2x80x2,7-bis-O-aroyl analogs. These analogs showed improved water solubility. The 2xe2x80x2-analogs showed in vivo activities in the same extent as paclitaxel and some even better.
Chemical stability is critical to the formulation and storage of any water soluble analog/prodrug of paclitaxel, since partial degradation to the poorly soluble parent drug is likely to lead to precipitation of paclitaxel. The enzymatic stability (in rat, human plasma or in vivo) is important in relation to the degradation of the prodrugs to paclitaxel or to an active metabolite of paclitaxel.
From the water soluble paclitaxel prodrugs described so far the pharmacological properties of the used solubilizing functionalities, which are released once paclitaxel is liberated have not been studied. It might be possible that these solubilizing moieties or their metabolites have some undesired and/or unknown side effects. The prodrugs described in this patent release after hydrolysis a non-toxic acid.
It is therefore an object of this invention to provide water soluble paclitaxel analogs/prodrugs, using a body innocuous solubulizing moiety, for the treatment of cancer.
A further object of this invention is to produce water soluble analogs of paclitaxel which possess (in vitro or in vivo) antitumor activity in the same extent as paclitaxel.
It is a further object of this invention to produce prodrugs of paclitaxel which are stable in aqueous solutions, but which upon hydrolysis at physiological (in vitro and/or in vivo) conditions exhibit the same or similar level of antitumor activity as paclitaxel.
The above and various other objects of the present invention are achieved by water soluble analogs/prodrugs having the following general formula: 
Wherein:
R1=C(O)CH2CH(OH)COOX
R2=H, C(O)CH2CH(OH)COOX,
X=H, Li, Na or any other pharmaceutically acceptable counterion.